Ultimate Chocolate Layer Cake

Position rack in center of oven and preheat to 350 degrees F.

Butter two 9-inch-diameter cake pans with 2-inch high sides. Line with parchment paper; butter paper. Dust pans with flour; tap out excess. Melt chocolate in top of double boiler over barely simmering water, stirring with rubber spatula until smooth. Cool to lukewarm, stirring often. Sift cake flour, baking soda and salt into medium bowl.

Using electric stand mixer and flat beater attachment, beat butter until fluffy, about 2 minutes. Gradually add both sugars, beating until creamy. Beat in eggs one at a time. Beat in vanilla. Scrape down sides of bowl with rubber spatula. Beat in melted chocolate; blend well. Add flour mixture alternately with sour cream in 2 additions, beating well after each addition. Add dissolved espresso; blend thoroughly.

Divide batter between prepared pans. Bake cakes until tester inserted into center comes out clean and cake just begins to pull away from sides of pan, about 30 minutes. Cool cakes in pans on racks 5 minutes. Turn out cakes onto racks; peel off parchment paper. Cool cakes completely.

DO-AHEAD TIP: Cakes can be baked 2 days ahead. Store airtight at room temperature.

Fill pastry bag fitted with large open star tip with 1/2 cup frosting; set aside. Slice each cake horizontally cake in half. Place one cake layer, flat side up, on 9-inch-diameter cardboard round. Spread 1/2 cup frosting over cake. Top with second cake layer, flat side down. Spread 1/2 cup frosting over top of cake. Repeat layering with remaining 2 cake layers and 1/2 cup frosting. Spread remaining frosting over top and sides of cake. Press chocolate shavings onto sides of cake. Pipe frosting from pastry bag decoratively around top edge of cake. Lightly sprinkle chocolate shavings over cake. Cover cake with cake dome. Refrigerate 20 minutes before serving.

DO-AHEAD TIP: Assembled cake can be made 3 days ahead. Keep refrigerated.